Many mobile service providers engage in cooperative “roaming agreements” with other mobile service providers. The advantage of such agreements is that not every service provider must expend the capital to provide cellular coverage in all geographic areas, particularly in rural areas. Roaming agreements therefore allow a mobile subscriber to roam between service provider networks without any interruption in service. One disadvantage of this arrangement, however, is that the trigger for roaming into a different network is bi-directionally symmetric (i.e., the same criteria is used to roam out of the “home” network as to roam back into it). This symmetry results in a possible overload of any one service provider network for its “home” subscribers as well as higher-than-necessary roaming charges to mobile subscribers when the subscriber is not returned to the “home” network as early as possible.
When a mobile terminal moves from a home network to another network, a handoff occurs. Mobile assisted soft handoff in conjunction with signal diversity combining is the method of relaying user communication signals on the transmission segment between a mobile terminal and a mobile switching center concurrently via a first and a second base station in the uplink and downlink direction and performing signal diversity reception at the mobile terminal and mobile switching center in order to enhance the user signal quality. This method is invoked by the mobile switching center when a mobile terminal communicating initially with a base station has moved into the overlapping coverage areas of this first base station and a second base station and has reported the availability of a sufficiently strong signal from this second base station to the mobile switching center. At no time instant during soft handoff in conjunction with signal diversity combining does the mobile terminal interrupt its communications with the mobile switching center. The mobile switching center typically deploys post-detection/decoding, selective combining of the digitally encoded speech frames.
In order to enable the reporting assistance of the mobile terminal during soft handoff initiation, all base stations may transmit, for example, a code division multiple access (CDMA) downlink reference signal, referred to as a pilot signal. Mobile terminals when roaming throughout the service area of the CDMA cellular telecommunications system periodically demodulate the pilot signals of the various neighboring base stations during an ongoing communication with a first base station and derive a corresponding pilot signal quality indication. This indication determines a ranked list of candidate base stations for handoff and is transmitted in the form of signaling information to the mobile switching center. It should be understood that also the first base station may perform continuously CDMA uplink signal quality strength and measurements and based upon these observations may give a soft handoff request indication to the mobile switching center.
Usually, soft handoff in conjunction with signal diversity combining is initiated by the mobile switching center if the mobile terminal reports that the pilot signal quality of a second base station in addition to that of the first base station is sufficiently good according to the predetermined thresholds made available to the mobile terminal and the mobile switching center as well as the second base station can obtain the required resources for the soft handoff transition. Subsequently, the mobile terminal will be instructed by the mobile switching center via the first base station by means of signaling to initiate a soft handoff and to commence signal diversity combining on the downlink.
Moreover, the mobile switching center initiates the additional relay of user signals via the second base station and commences diversity combining of the user signal in the uplink direction. Both participating base stations invoke autonomously the previously mentioned closed loop power control method. The mobile terminal sets its CDMA transmit power to the minimum of the two commanded power levels in order to reduce excessive CDMA interference with the other communication links.
Finally, when the mobile terminal is firmly established within the area of the second base station and the pilot signal received from the first base station has weakened sufficiently according to the predetermined thresholds made available to the mobile terminal it will report this condition to the mobile switching center, which in turn decides to terminate the soft handoff with signal diversity combining and will use subsequently only the second base station for maintaining the CDMA communications. One disadvantage of this arrangement is that the trigger for roaming into a different network is bi-directionally symmetric.
Thus, a need exists for an improved telecommunication network that optimizes roaming between service providers by minimizing the duration of off-network roaming.